Electrostatic discharge (ESD) causes damages to semiconductor components.
A conventional electrostatic discharge protection structure utilizes a silicon controlled rectifier (SCR), as shown in FIG. 1. The structure of the SCR includes a PN junction structured made of a p-type substrate and an n-type well region formed by n-dopants in the p-type substrate. The p-type substrate and the n-type well region are doped with dopants to form n+ and p+ regions therein, respectively, as contact regions. Heavily doped counter-doped p+ and n+ regions are formed between the contact regions and at a lateral junction between the substrate and the well region. The n+ contact region and the p+ counter-doped region in the n-type well region are electrically coupled by a metallic lead line to form an anode. Similarly, the p+ contact region and the n+ counter-doped region in the p-type substrate are electrically coupled by a metallic lead line to form a cathode. The n+ counter-doped region is equivalent to the emitter of an NPN transistor, and the p+ counter-doped region is equivalent to the emitter of a PNP transistor. The NPN transistor and the PNP transistor are coupled together to form the SCR.
During normal operation (the voltage at the anode being higher than that at the cathode), a diode formed between the n-type well region and the p-type substrate is reverse-biased and the operating voltage is below the breakdown voltage of the diode, thus the SCR is not triggered and the current is small. However, during a surge of electrostatic discharge, the voltage at the anode reaches the breakdown voltage. This results in impact ionization to generate a large amount of electron-hole pairs that move toward the n-type well region and the p-type substrate under the effect of electric field. The movement of the holes into the p-type substrate causes a potential drop, resulting in the connection between the base and emitter of the PN junction to allow electrons to flow from the emitter to the base. The same mechanism also causes carrier drift in the n-type well region to cause holes to flow from the emitter to the base. The drift of electrons and holes cause the electric potential to increase or decrease, thus further enhancing the degree of impact ionization. This bootstrap process allows the SCR to form a current channel suitable for electrostatic discharge with low resistance and for large amount of current.
However, this type of conventional electrostatic discharge protection structure has the problem of high trigger voltage (or threshold voltage), this may result in ESD damage for components with thinner oxide layer and higher susceptibility to damage due to the higher trigger voltage.